1. Field of the Invention
The present invention relates generally to semiconductor manufacturing. More specifically, the present invention pertains to a reference sample mounted on a stage to allow convenient access to the reference sample during collection of a reference sample spectrum.
2. Description of Related Art
During the manufacturing of semiconductor devices, a film (e.g., an epitaxy or an oxide layer) is formed on a wafer surface. After the film is formed on the wafer, a characteristic of the film, such as film thickness is measured to determine if the desired film thickness is achieved. A flow chart of a prior art method of determining film thickness is illustrated in FIGS. 1A-B. Beginning with FIG. 1A and step 101, an operator places a cassette of unprocessed wafers into a processing tool. The processing tool typically has several chambers and each chamber performs a different function. For instance, the film is formed on the wafer in a processing chamber and in some cases, the wafer is then cooled in a holding or cooling chamber before it is removed from the processing tool. A film of a predetermined target thickness is grown/formed on the wafers in a processing chamber of the processing tool in step 103. In step 105, the operator removes the cassette of processed wafers from the processing tool and brings the cassette of processed wafers to a film thickness measurement tool. Because the operator removes the wafers from the processing tool before measuring the film thickness formed on the wafers and then measures the processed wafers in a separate measurement tool, this is an off-line process.
The operator, however, often places a reference sample in the film thickness measurement tool to obtain a reference spectrum in step 107, before measuring the spectrum of a processed wafer. Once the operator has obtained a reference spectrum for the reference sample, then in step 109, the operator removes the reference sample from the film thickness measurement tool. In step 111, the operator places one of the processed wafers (e.g., a first processed wafer) into the film thickness measurement tool to obtain a spectrum for that (e.g., first) processed wafer. The film thickness measurement tool typically uses infrared radiation that is directed toward the processed wafer and then reflected off the processed wafer to collect a spectrum for that particular wafer. In step 113, a computer system, which has already stored the reference sample's spectrum, compares the reference sample's spectrum to the (e.g., first) processed wafer's spectrum. In FIG. 1B and step 115, the comparison between the reference sample's spectrum and the first processed wafer's spectrum yields a first differential value that helps determine the thickness of the film on the (e.g., first) processed wafer.
In step 117, the operator removes the measured (first) processed wafer from the film thickness measurement tool. The operator, in decision diamond 119, must decide whether she or he is finished measuring processed wafers. If the answer is "yes", then the process ends. If on the other hand, the answer is "no", then the next step is decision diamond 121, where the operator must decide whether to remeasure the reference sample to collect another reference spectrum. If the operator decides to remeasure the reference sample, then the process returns to step 107, where the operator again places the reference sample in the film thickness measurement tool. The process repeats itself again from steps 107 through step 117 but with the replacement of a "second processed wafer" for a "first processed wafer." Step 107 through step 117 are repeated for each measured processed wafer. A spectrum for a particular processed wafer and a reference spectrum are necessary to determine the film thickness on that particular processed wafer.
If the operator decides not to remeasure the reference sample, then in decision diamond 123, the operator decides again whether to measure another processed wafer. If the answer is "no", the process ends. But if the answer is "yes", then the process returns to step 111. Once again, in steps 113, 115 and 117, "a (first) processed wafer" should be replaced with "a (second) processed wafer." The operator must place the reference sample in the film measurement tool each time before collecting the reference sample's spectrum and then remove the reference sample each time before measuring the processed wafer. Thus, an operator must handle the reference sample numerous times during the film thickness measurement process.
A process, similar to the flow chart shown in FIGS. 1A-1B, can be used to measure another film characteristic, such as film resistivity. Instead of a film thickness measurement tool, the operator uses a film resistivity measurement tool. The process of measuring film resistivity, however, also suffers from the same drawbacks.
Therefore, the prior art method of measuring the reference sample suffers from several disadvantages. First, there is a decrease in throughput as the placement of a reference sample in the measurement tool displaces a processed wafer. Second, increased operator handling of the reference sample and wafer leads to a greater incidence of handling or contamination errors. This is because the operator must handle the reference sample and wafer every time they are placed in the measurement tool and every time they are removed from the measurement tool. Third, there is a throughput decrease because of the impact on the processing tool or computer system each time the reference sample must be moved when a reference spectrum is collected.
Thus, it is desirable to have a method of measuring a reference characteristic without moving the reference sample on and off a stage in a measurement tool each time the reference sample is measured.